Jon has also mentioned in the past that the GUI toolkit is custom-built. He codenamed it "Hydra". Back in 2010 he announced he would make a post on the technology underlying Sublime Text, but I have the feeling ST2 snowballed on him :) Maybe he'll find time to write about the ST3 implementation some day, but it will be hard to keep up with the codename's coolness. For the extremely curious, you might be able to dig up a longish forum post of his on character rendering.